Pyoverdin and pyochelin are siderophores for Pseudomonas aeruginosa. These compounds are involved in iron solubilization and iron transport. Pyocyanin is also involved in iron metabolism due to its ability to reduce Fe(III) for transport. This bacterium is an opportunistic pathogen and causes special hazards in nosocomial infections. A model infection in mice which are compromised in the phagocytic cell and nutritional immune defense mechanisms, has allowed the study of the growth of P. aerugionsa in tissue. An isolate originating from an avirulent P. aeruginosa was selected for its ability to grow in these mice and expresses virulence which is based upon iron utilization during infection. Comparisons between the virulent and avirulent bacteria will allow elucidation of differences in iron metabolism. These comparisons will use 55Fe binding and uptake to measure differences between isolates in siderophore production and metabolism; in reactions between iron-siderophores and binding proteins on P. aeruginosa; and in the competition of these processes with transferrin for iron. Effects of the antibody response to the iron uptake systems will be examined during this comparison. Isolates from human infections will be included in this comparison to determine whether the same or different alterations in iron metabolism also appear during the growth of P. aeruginosa in human tissue. Changes in the respiration of P. aeruginosa have been observed during growth in oxygen-limitation. These alterations in components of the respiratory system and affinity for oxygen can also be measured in infecting cells to determine whether oxygen limitation is a part of the selective pressure of mammalian tissue exerted on this obligately aerobic bacterium.